Various types of hydrocarbons such as natural gas, coal gas (COG), liquefied petroleum gas (LPG) and naphtha, etc., are used as raw materials in steam reforming processes. These hydrocarbons generally contain sulfur. This sulfur poisons catalysts used in the steam reforming process or other processes, and thus lowers the catalytic activity. Accordingly, it is necessary to subject the raw material to a desulfurization treatment in advance.
Conventionally, a typical desulfurization method performed prior to the steam reforming of hydrocarbons is a hydrodesulfurization method comprising the steps of subjecting the organic sulfur contained in the hydrocarbon raw material to hydrogenolysis with using a Co—Mo type or Ni—Mo type catalyst, and removing the hydrogen sulfide thus produced by adsorption on zinc oxide.
However, there are problems in such a conventional method. Specifically, in the hydrodesulfurization process, a certain amount of organic sulfur, especially organic sulfur that is difficult to decompose, such as thiophene, etc., may pass through without being adsorbed on the zinc oxide.
Furthermore, since there are equilibria indicated by (for example)ZnO+H2S=ZnS+H2OZnO+COS=ZnS+CO2 in the adsorption, the amounts of H2S and COS, etc., likewise do not fall below fixed values. This tendency is especially conspicuous when H2O and CO2 are present. Furthermore, in cases where the desulfurization system is unstable at the time of start-up or shut-down, etc., of the apparatus, sulfur may also be scattered from the hydrodesulfurization apparatus and adsorption-desulfurizing agent, so that the sulfur concentration in the purified product is increased. Accordingly, the desulfurization process in current steam reforming processes must be controlled at a level which is such that the sulfur concentration in the hydrocarbon after purification is approximately 0.1 ppm.
On the other hand, Ni or Ru catalysts, etc., are used in steam reforming processes. It is known that sulfides are formed on the surfaces of these metals even at low sulfur concentrations of 1 ppm or less. For example, as has been demonstrated by the research of McCarty et al. (McCarty et al.; J. Chem. Phys., Vol. 72, No. 12, 6332, 1980; J. Chem. Phys., Vol. 74, No. 10, 5877, 1981), since the sulfur adsorbing powers of Ni and Ru are extremely strong, even in cases where the sulfur content contained in the raw material is approximately 0.1 ppm, the surfaces of Ni and Ru catalysts are almost completely covered by sulfur in an equilibrium state (sulfur coverage rate of 0.8 or greater). Namely, steam reforming catalysts are extremely sensitive to sulfur, so that such catalysts show a drop in catalytic activity in the presence of even a small amount of sulfur. This means that the sulfur poisoning of steam reforming catalysts cannot be sufficiently prevented at the current level of hydrocarbon desulfurization.
Especially in the case of substitute natural gas manufacturing processes in which methane-rich gases are prepared, since the processes are performed at a low temperature, sulfur is readily adsorbed on the catalyst. Processes are even more sensitive to low concentrations of sulfur. Furthermore, even in steam reforming processes, which are performed at a higher temperature, low concentrations of sulfur have a serious effect in the case where the size of the reaction apparatus must be reduced, as in fuel cell reformers.
Accordingly, in order to prevent sulfur poisoning of the catalyst in subsequent processes and improve the economy of the overall process, it is extremely desirable to minimize the sulfur content in the raw material.
From such a standpoint, Japanese Unexamined Patent Publication No. H1-123627 and Japanese Unexamined Patent Publication No. H1-123628 disclose a method for manufacturing a copper-zinc type desulfurizing agent and a method for manufacturing a copper-zinc-aluminum type desulfurizing agent. When these desulfurizing agents are used, the conspicuous effect of a reduction of the sulfur concentration in the raw material to 1 ppb or less is achieved. However, a large amount of these desulfurizing agents must be used if it is desired to maintain a high level of desulfurization over a long period of time.
On the other hand, it is known that iron and nickel are superior in terms of sulfur adsorption capacity, and that these metals show a superior performance as desulfurizing agents. Accordingly, these metals have been used as desulfurizing agents in several processes.
However, there is a serious impediment to using iron type desulfurizing agents or nickel type desulfurizing agents as is in the desulfurization of steam reforming processes. Specifically, desulfurization in ordinary steam reforming processes is performed in the presence of hydrogen, and this hydrogen is supplied by recycled gas from the outlet port of a reformer. This recycled gas contains CO and/or CO2 as well as hydrogen. Accordingly, in the presence of an iron type or nickel type desulfurizing agent, a reaction of the hydrogen with CO and CO2 (methane forming reaction) occurs, which is accompanied by the problem of a large amount of heat generation.
Japanese Unexamined Patent Publication No. H2-204301 discloses a method in which the abovementioned methane forming reaction is suppressed by causing the raw material to contact a hydrodesulfurization catalyst and a hydrogen sulfide adsorbing agent, and then introducing steam and using a nickel type desulfurizing agent in a steam atmosphere. In this method, however, the following problems arise: specifically, a steam introduction line is required not only for the steam reforming reactor but also for the desulfurization vessel, and as a result of the introduction of steam into the desulfurization vessel, the inherent desulfurization performance of the nickel type desulfurizing agent cannot be fully utilized.
Furthermore, the use of nickel type desulfurizing agents in the absence of hydrogen has also been reported. However, hydrogen is essentially necessary for the decomposition of organic sulfur compounds and the separation of hydrocarbons that contain no sulfur. Supposing that organic sulfur compounds are decomposed in a state in which no hydrogen is present, the deposition of carbon tends to occur on the surface of the nickel desulfurizing agent, leading to a rise in the differential pressure or blocking of the desulfurizing layer, and so forth, over the long term.